Such methods of modifying the region displayed within a digital image, for example in order to zoom out, are commonly used when it is desired to navigate in a digital image of large size, that is to say to view certain parts of the digital image on a display device by running through the digital image in accordance with the wishes of a user sent to the display system by an interface of mouse or keyboard type.
The digital image as a whole is described by data which may be stored locally by the display system (for example on a hard disk) or remotely, for example on a remote server, in which case the data will have to be downloaded before use by the display system. Moreover, the data are in general encoded in compressed form in order to facilitate their storage and transmission, which however involves a decoding operation at the time it is desired to view (i.e. display) the corresponding image.
The obtainment of displayable data (i.e. requiring only minimal processing by the display system) is thus not in general immediate, for example due to the time of downloading and/or decoding mentioned above.
Thus, when digital images of large size are manipulated (for example in case of navigation within the digital image), the displayable data are determined (i.e. after possible downloading and decoding) with respect only to the regions of interest for the user, i.e. in practice with respect to the region of the image to be displayed at a given time.
When the region to display in the digital image is modified (for example on request by the user of the display system), it is then necessary to obtain and determine the data with respect to the new region to display.
In order to improve the visual reproduction of the transition between the two regions to display, it is however sought to present the user with intermediate views which simulate the movement of a camera from the former region to display to the new region to display.
To perform this transition phase with sufficient fluidity and rapidity with respect to the user's request, it is necessary to have access to information describing the digital image externally of the former region displayed without waiting for the display system to know all the data relative to the new region to display.
To perform this type of transition when the user requests a zoom out during navigation (i.e. he requests the display of a new region of the digital image including the region displayed previously), it has already been proposed to store locally in the display system displayable data used at the preceding steps of the navigation and to use these stored displayable data to display intermediate views (of quality possibly lower but immediately available) during the transition.
In practice, those displayable data are for example constituted by bitmaps as is the case in the ZOOMIFY software of which the general features are given for example by the document “Zoomify Technology & Products White Paper” dated October 2003 and available from Zoomify, Inc., Santa Cruz, Calif.
According to this solution, when the user requests a zoom out from a first region of the digital image, the software searches for a bitmap stored locally in the display device and which would describe a second region including the first region and centered thereon, which appears natural as regards giving the user the impression of a zoom out. The bitmap which describes the second region is then used to generate intermediate views with a response time that is perfectly satisfactory since no downloading or decoding operation is necessary.
The bitmap which describes the second region may however have a mediocre resolution, for example when the stored bitmaps with a good resolution (for example immediately below that of the first region) are not centered on the first region and that it is therefore necessary to use a low resolution bitmap to be able to extract views including the first region and centered thereon.
The resolution of the intermediate views is thus not optimum and the quality of the transition phase simulating the zoom out may show this.